The present invention relates to a subscriber's circuit which controls a speech signal in a time division digital exchange, that is, a time division switching system which performs switching of speech channels by a highway switch inserted in a highway.
In a switching system of the type, a processor periodically scans subscriber's circuits to detect hook information and dial information. In this case, control commonly referred to as channel control is mostly carried out at a relatively low speed. However, since this periodic scanning calls for real time processing, the control operations are performed for each level of control. This tends to make the control operations complicated. Especially in recent years, a system based on a stored program control system using a microprocessor or the like has come into wide use, so that the system is now being enhanced in function. At present, the actual complication of control exceeds the progress of the capabilities of the microprocessor.
In the above switching system, a click dial tone is leaked out at the start of a dialing operation.
To avoid this, it is necessary to introduce a special high-speed routine in the operation of software control, making the control operations more complex than ever.
In recent years, the microprocessor technology has come into use for four-wire switching system, and a stored program control system has prevailed as its control system. In general, control of the exchange includes an operation which must be controlled at a high speed and in real time, such as the detection of hook information and dial information, and an operation which may be carried out at a low speed, such as call processing control. On account of this, the stored program control employing the microprocessor usually utilizes level of control, and hence multi-levels of control. This makes the control of the exchange equipment extremely complicated and inflexible. Further, the complexity of exchange equipment the function of which is enhanced more and more, such as a key telephone system, exceeds the progress of the capabilities of the microprocessor.
With such a background, the microprocessor technology is now attracting attention in the persuit of simplification and flexibility (extendability) of software.
In the above four-wire switching systems, while a short duration of dial tone is also leaked out at the start of a dial operation, this problem has not yet been resolved by simple and economical circuitry or software.